This past Field Day the club tried to use a folded loop that
just didn't work.All contacts were made on my portable antenna. The
club asked me to make something for them that would work on 75, 40, and 20
meters.

Let's skip right to the conclusion. I used this antenna to
check in with my favorite regional net on 75 meters with my Icom IC-718
operating at 100 w SSB from a RBC-6 sealed lead acid ("gel cell") battery.
Net control and assistant net controls reported that I sounded good, one
giving a signal report of "10 over" from nearly 400 miles
away.

This antenna will work on 40 and 20 meters without tuner,
and with a little assist from a tuner will work on 17 and 15 meters,
too.

Here it
is:

This is a Field Day / Portable / Emergency antenna I just
finished trimming.An antenna analyzer made for easy
trimming.

This is a multiband inverted V dipole. The club president
and I had discussed bands that needed to be covered, so it was decided
that 20 meters, 40 meters, and 75 meters were most important. Anything
else is extra.

I made my calculations, cut my wire to length, etc.
Left a few feet on the ends so that it could be let out.

The 4 legs
are all about the same overall length. 40 meter legs go one way, and the
75/20 meter legs are at right angles. The wires double as the guys, with
insulators out at the ends, and then 550 parachute cord going on out to
tent stakes... or anything else you can find to tie to.

The 75/20
legs have coils in the middle of each leg. These are placed at about where
the ends would be if the wires were cut for 20 meters only... traps or
chokes. They also act as loading coils so that the overall length will
work for 75 meters.

When
dipoles are this low (about 22'), proximity to the ground lowers the
antenna's impedance, which causes havoc with SWR. To counter this, like
the Buddipole, the feedpoint is just off center, enough to bring up the
impedance a bit closer to 50 ohms. The legs connected to the "hot side" of
the coax are a little longer, and the coil on the 75/20 meters leg has a
few more turns on the coil than calculated. The legs connected to the
"shield side" of the coax are a little shorter, and the coil on that
shield side 75/20 meters leg has a few fewer turns than calculated. This
off center feed allowed for a minimum SWR of 1.3 to 1.4:1, which could not
have been achieved otherwise.

Let's see if this theory works
out...

20 meters resonated too high, needed
lengthening.

Due to the
way this antenna is made it was decided to lengthen the 20 meters segments
first by unwinding a turn from the inner side of one coil. The coil
former, 2" pvc pipe, has J-slots cut in each end so that wire may be let
out or taken in.

Letting out one turn, lengthening that side by
about 7", brought resonance down some, but not enough. Letting out one
more turn on that same side resulted in these figures:

After these changes all three bands were
checked again to make sure nothing else needed to be fine tuned.

15
meters was found to be in the 3.5:1 to 3.0:1 range. A tuner can take care
of that quite easily.

Loading coil. These coils are close wound on 2" ID pvc with
14 ga insulated wire. There are J slots on each end to allow adding or
subtracting turns of wire.

Once the coil tuning was settled it was wrapped
with 3M Electrical Tape.

Center insulator made from cheap Wallyworld kitchen cutting
board.

100' of RG-8X coax was used, which is approximately 1/2
wavelength at 80 meters. Also, there are five Palomar Engineers FSB-1/4
ferrite snap on beads on the coax up by the feedpoint. These act as a
"choke balun".

Base, PA Speaker tripod. The mast is 5 sections of
fiberglass pole sections. These are military surplus camo net poles, an
Ebay item.

Wire used is 14 ga stranded insulated wire, the 500' roll
for $25 from Home Depot.

I had a lot of black parachute cord, but
will buy some more orange for the end rope on this. Also, my plastic tent
stakes are green. Difficult to see in the dark. I'm going to get some
bright yellow tent stakes.

The center insulator is cut from a cheap
plastic kitchen cutting board from Walmart. A half inch dia hole was
drilled in it, and a SO-239 socket screwed to it. The top of the socket
was sealed with Aleene's 7800 adhesive.

There is approximately 3"
of wire from the socket to each of the four holes, or tie points for the
wire legs.

The two 40 meters segments go one way, and the two 75/20
meters segments are perpendicular to the 40 m.

40
meters segments:

Measurements are from the holes, or tie points on the center
insulator out to the hole in the ceramic "dog bone" insulators on the
ends.

75 and 20 meters
segments:

This 75 / 20 meters section is wired parallel to the 40
meters section, only these legs are stretched out at right angles to the
40 meters legs.

Measurements for the inner 20 meters segments are
from the tie points on the center insulator out to the first turn on the
coils. The measurements for the outer wire segments are from the last turn
of the coil on out to the hole in the ceramic "dog bone" insulators on the
ends.

The coil formers are 2" ID PVC pipe, which is 2.375" OD. I
have holes drilled in the pipe to secure the ends of the coils with
ty-wraps. After tuning was deemed finished, the coils were wrapped with
Scotch 3M electrical tape. I find Scotch electrical tape does not turn
gummy and fall off with age. Good quality tape is worth the money. I made
the coil formers with "J-slots" on the ends, to allow more turns to be
taken off or added for tuning. Now that I have the final measurements that
will not be necessary when making future copies of this
antenna.

Due to the diameter of the 14 ga insulated wire, I was
able to get right at 9 turns per inch, close wound, that is, turns
touching each other. I used this 9 turns per inch figure in an online coil
design calculator. This gave a nice repeatable build on the coils, and a
length to diameter ratio of about 1.75:1, which is right in the middle of
the suggested design ratios of 1.5:1 and
2:1.

Other
notes:

There are five Palomar Engineers FSB-1/4 ferrite snap
on beads placed on the RG-8X coax near the feedpoint. This forms a "choke
balun" to stop RF on the shield. Similar snap on beads may be purchased
from Ham City listed with the coax.

The apex of the antenna is at
approximately 22'. This includes the tripod base and 5 fiberglass mast
pieces. These are the common military surplus fiberglass camo net poles
sold at hamfests and on eBay for use as antenna masts.

The only
drawback of the coils is that the 75 meters bandwidth is narrower than if
the antenna were full length. However, the antenna still covers the
desired portion of the band with good SWR, and a little more using the
tuner.* (see note at bottom of this post)

But the use of coil
loading and the overall length of this 75 meter antenna (appx 50% of full
size) causes no noticeable drop in signal. I still got good signal reports
from others I regularly talk to, so they know what I usually sound like
with my full size dipole.

While this is not a permanent antenna for
me, this might possibly help others fit 75 or 80 meters into a small
yard.

One added note: While many say coax length does not
matter, during initial testing with an analyzer I was getting good,
consistent readings on 40 and 20 meters, but the 75 meters SWR readings
were squirrely. Then it dawned on me... 50' of coax. Considering velocity
factor, that is very close to 1/4 wavelength at 80 meters. The next time I
tested it I used 100' of coax, and that works out to approximately 1/2
wavelength at 80 meters. SWR readings were consistent and
stable.

Lesson learned: The old "conventional wisdom" to use 1/2
wavelength of coax got to be "conventional wisdom" for a good reason. I
related this to one of our old club gurus and he just smiled and said,
"Toljaso." Length does matter, so use 93' - 100' of coax with this
antenna.

????-

* So, a thought occurred to
me... What if you do want to go lower in the 75 meter band? Why not put
some sort of connector near the ends, down by the insulators? Then clip on
some added pieces of wire, 1' long each, or 2' long each, and string them
on past the insulator, perhaps securing them to the parachute cord end
ropes with tie wraps? That would drop the resonant frequency to a lower
portion of that band.

Update!New Additional Mods added to antenna base below!
09-28-2012

Added 1" split rings (like for
car keys) and snap hooks

New Mod to Field
Day Antenna done with Anderson Power Pole
connectors

Measurements of wire lengths are now from the Powerpoles to
the coils or end insulators.

Note that you may also download a pdf file of the antenna
construction with mods

Errecting the Field Day /
Emergency / Portable
AntennaSlip round insulator over top tenon of the top fiberglass mast
piece.

Attach coax. Use
75' - 100' of RG-8X. Do not use 50', or it won't work on 75
m.

Clip the four wire
elements to the appropriate rings on the center insulator. Observe colors
and stripes on the connectors. The two plain wires will be opposite each
other, with red to center, black to shield. The two coil loaded wires will
be opposite each other with red-yellow to center, black-yellow to
shield.

Spread out the
wires fully, then bring in the ends of the ropes about 10'--12' and stake
to ground. I suggest bright yellow tent stakes to help prevent tripping
hazard.

Raise center mast
vertically, placing new sections in one at a time from the bottom. Use a
total of 5 sections of mast. Place on tripod.

Now go around and
adjust stake position, wire/rope
tension.

You may want to add
ribbons of Caution Tape to prevent
clothes-lining.

Will work 75 meters
(resonant about 3.920 mhz), 40 m, 20 m, with little to no touch up from
the tuner. Will work 17 m, 15 m, and 10 m with light touch up from tuner.
Have not tried it on 6 m, but suspect it might work there, too. Let me
know.

We have worked,
from the Louisiana Gulf Coast, from coast to coast on Field Day, NY to CA,
on 20 m and 15 m. Worked all over the Gulf Coast states, FL to TX on 40 m.
At night I've done a regional net, LA, MS, TX, AR, OK, on 75 m at 100 w
running off batteries, with good signal
reports.

75 meters, due to
the coil loading, is somewhat narrow. A tuner
helps.

There you have
it...Enjoy and have fun! AE5JU - Paul

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email address if you have questions!

Feedback:

Just wanted to pass a note along.

Had the opportunity to work the digital side of station K5R
- Katrina / Rita 5th Anniversary. Fun and rewarding. To the point I had to
bring all the gear I was going to use to the SELARC club station. I
decided to build a copy of your field day antenna as described at

http://www.hamuniverse.com/ae5jufielddayantenna.htmlIt worked like a champ. I tuned it in the
backyard and hoped it would work on the roof of the Ponchatoula Community
Center. Once set up on the roof I checked the SWR with a MFJ antenna
analyser. 20m was spot on. 40m was a bit off at 2.2 but good enough. Not
worth the hassle of going back up there to tweak it. The tuner handled 30m
so I could use that also.

Ended up with 60 QSOs from 28 states and 3 DX entities.
Thanks for the antenna design.